📚 Breathing and Gas Exchange | 呼吸与气体交换
In Edexcel International GCSE (9-1) Biology, Chapter 3 focuses on breathing and gas exchange. It explains how air is moved into and out of the lungs, how oxygen and carbon dioxide are exchanged at the alveoli, and how the respiratory system adapts to meet the body’s demands.
在爱德思国际 GCSE（9-1）生物第 3 章中，重点是呼吸与气体交换。本章解释空气如何进出肺部、氧气和二氧化碳如何在肺泡处交换，以及呼吸系统如何适应身体的需求。
1. Breathing vs Respiration | 呼吸与细胞呼吸的区别
Breathing, or ventilation, is the physical process of moving air into and out of the lungs. It does not release energy from food.
呼吸（通气）是空气进出肺部的物理过程，它本身并不从食物中释放能量。
Respiration is a chemical reaction that releases energy from glucose inside cells. It uses oxygen and produces carbon dioxide as a by-product.
细胞呼吸是细胞内从葡萄糖释放能量的化学反应，它消耗氧气并产生二氧化碳副产物。
Students often confuse these terms: breathing supplies the oxygen for aerobic respiration and removes the carbon dioxide it produces.
学生常混淆这两个术语：呼吸为有氧呼吸提供氧气，并排出有氧呼吸产生的二氧化碳。
2. The Human Respiratory System | 人体呼吸系统结构
Air enters through the nose or mouth and passes through the larynx, trachea, bronchi, bronchioles, and finally reaches the alveoli.
空气从鼻或口进入，经过喉、气管、支气管、细支气管，最后到达肺泡。
The trachea and bronchi contain C-shaped rings of cartilage that hold the airways open and prevent collapse.
气管和支气管含有 C 形软骨环，使气道保持张开并防止塌陷。
Goblet cells secrete mucus to trap dust and microorganisms, while ciliated epithelial cells waft the mucus up to the throat.
杯状细胞分泌黏液以黏附灰尘和微生物，纤毛上皮细胞则将黏液向上扫至咽喉。
The lungs are enclosed in the rib cage, and the diaphragm separates the thorax from the abdomen.
肺被胸廓包绕，膈肌将胸腔与腹腔分隔开。
3. Ventilation: Inspiration and Expiration | 肺通气：吸气与呼气
During inspiration, the external intercostal muscles contract and the diaphragm contracts and flattens. This increases the volume of the thorax.
吸气时，外肋间肌收缩，膈肌收缩变平，使胸腔容积增大。
The increased volume lowers the pressure in the lungs below atmospheric pressure, so air flows down a pressure gradient into the lungs.
容积增大使肺内压低于大气压，于是空气沿压力梯度流入肺部。
During expiration, the external intercostal muscles and diaphragm relax. The thorax volume decreases, raising lung pressure above atmospheric pressure, so air is forced out.
呼气时，外肋间肌和膈肌放松，胸腔容积减小，肺内压高于大气压，空气被排出。
Forced expiration, as in coughing or sneezing, uses the internal intercostal muscles and abdominal muscles.
用力呼气（如咳嗽或打喷嚏）会动用内肋间肌和腹部肌肉。
4. Pressure-Volume Model of the Thorax | 胸腔压力-容积模型
Use the simple pressure law to explain ventilation: at constant temperature, an increase in volume causes a decrease in pressure.
可用简单压力定律解释通气：温度不变时，容积增大导致压力降低。
P₁V₁ = P₂V₂
Although the thorax is not a sealed gas container, the linked rib cage, diaphragm, pleural membranes and lung tissue behave like a bell-jar model.
尽管胸腔不是密封气体容器，但胸廓、膈肌、胸膜和肺组织的联动与钟形罩模型相似。
In the bell-jar model, pulling the rubber sheet down increases volume and draws air into the balloons; pushing it up has the opposite effect.
在钟形罩模型中，下拉橡胶膜增大容积，空气被吸入气球；向上推则相反。
5. Alveoli and Gas Exchange Surface | 肺泡与气体交换面
Alveoli are tiny air sacs at the ends of bronchioles. Each lung contains about 300 million alveoli, giving a huge total surface area for diffusion.
肺泡是细支气管末端的小气囊。每个肺约有 3 亿个肺泡，为扩散提供巨大的总表面积。
Oxygen diffuses from the alveolar air into the blood through the alveolar wall and capillary wall. Carbon dioxide diffuses in the opposite direction.
氧气经肺泡壁和毛细血管壁从肺泡空气扩散入血；二氧化碳沿相反方向扩散。
The alveolar wall is one cell thick and is lined by a film of moisture. The capillary wall is also one cell thick, so the diffusion distance is very short.
肺泡壁仅一层细胞，并衬有一层水膜；毛细血管壁也仅一层细胞，因此扩散距离极短。
Each alveolus is surrounded by a dense capillary network. Blood continuously removes oxygen and supplies carbon dioxide, maintaining steep concentration gradients.
每个肺泡周围有丰富的毛细血管网。血液不断带走氧气并带来二氧化碳，维持陡峭的浓度梯度。
6. Adaptations of the Gas Exchange Surface | 气体交换面的适应特征
Large surface area is provided by millions of alveoli; this increases the rate of diffusion.
数百万肺泡提供巨大的表面积，从而提高扩散速率。
Thin walls reduce the diffusion distance; alveolar and capillary walls are each one cell thick.
薄壁缩短扩散距离；肺泡壁和毛细血管壁各为一层细胞厚。
Moist alveolar lining dissolves oxygen and helps it pass through the cell membranes.
潮湿的肺泡内壁可溶解氧气，帮助其穿过细胞膜。
A dense capillary bed constantly carries blood past the alveoli, keeping the concentration gradient steep.
密集的毛细血管网不断使血液流经肺泡，维持陡峭的浓度梯度。
7. Inhaled and Exhaled Air | 吸入气与呼出气的比较
The table compares approximate percentages of gases in inhaled and exhaled air.
下表比较吸入气与呼出气中气体的近似百分比。
| Gas | Inhaled air | Exhaled air | Reason for change |
|---|---|---|---|
| Nitrogen | about 78% | about 78% | not used by cells |
| Oxygen | about 21% | about 16% | used in aerobic respiration |
| Carbon dioxide | about 0.04% | about 4% | produced in aerobic respiration |
| Water vapour | variable | saturated | evaporation from wet lung lining |
| Temperature | ambient | body temperature | air warmed in the lungs |
Nitrogen percentage stays much the same because nitrogen is not used in respiration.
氮气比例基本不变，因为呼吸作用不消耗氮气。
Oxygen decreases and carbon dioxide increases because aerobic respiration consumes oxygen and produces carbon dioxide.
氧气比例下降，二氧化碳比例上升，因为有氧呼吸消耗氧气并产生二氧化碳。
Exhaled air is warmer and saturated with water vapour because the gas exchange surface is warm and moist.
呼出气温度更高且水蒸气饱和，因为气体交换面温暖而湿润。
8. Investigating Breathing and Exercise | 呼吸与运动的实验研究
Breathing rate can be measured in breaths per minute by counting chest rises over 30 seconds and multiplying by two.
呼吸频率可通过计数 30 秒内胸廓起伏次数再乘以 2 来测量，单位为次/分钟。
Exercise increases breathing rate and depth because muscles need more ATP and produce more carbon dioxide.
运动会提高呼吸频率和呼吸深度，因为肌肉需要更多 ATP 并产生更多二氧化碳。
This is a homeostatic response: the brain detects increased carbon dioxide in the blood and increases ventilation.
这是稳态反应：大脑检测到血液中二氧化碳升高，从而增加通气量。
The bell-jar model demonstrates the mechanical action of the diaphragm but does not show gas exchange at the alveoli.
钟形罩模型展示膈肌的机械作用，但不能显示肺泡处的气体交换。
9. Smoking and the Respiratory System | 吸烟与呼吸系统
Tar in tobacco smoke is carcinogenic and can destroy the cilia and cause chronic bronchitis and emphysema.
烟草烟雾中的焦油具有致癌性，可破坏纤毛，并引起慢性支气管炎和肺气肿。
Nicotine is an addictive drug that increases heart rate and blood pressure, raising cardiovascular risk.
尼古丁是一种成瘾性药物，会使心率和血压升高，增加心血管风险。
Carbon monoxide binds to haemoglobin more strongly than oxygen, reducing the oxygen-carrying capacity of blood.
一氧化碳与血红蛋白的结合力强于氧气，从而降低血液携带氧气的能力。
Damage to cilia allows mucus and bacteria to accumulate in the airways, causing smoker’s cough and infection.
纤毛受损使黏液和细菌在气道内积聚，引起吸烟者咳嗽和感染。
| Component | Main effect |
|---|---|
| Tar | carcinogen; damages cilia |
| Nicotine | addictive; raises heart rate and blood pressure |
| Carbon monoxide | binds haemoglobin; reduces oxygen transport |
10. Key Concepts and Exam Tips | 核心概念与考试建议
In exam questions, always state the path of air in the correct order: trachea → bronchi → bronchioles → alveoli.
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